Cement raw material containing kerogen therein, such as oil shale or material having fuel with high amounts of volatiles, have not heretofore been processed successfully in suspension and grate preheater kiln systems. This is true because of the fact that the kerogen in the raw material produces a tremendous amount of unburned combustible volatiles that pass with the process gas. The grate preheater kiln system is not designed to handle these combustible gases and to efficiently recover their heat for use in the process or associated process. The present cost of fuel has made it economical to process cement raw materials containing kerogen or other high volatile fuels. Thus, cement raw materials such as is found in some of the western states of our nation can now be utilized since it is economical to process these materials.
Examples of systems which exemplify grate preheater kiln systems are disclosed in U.S. Pat. No. 2,925,336, issued to Stowasser; U.S. Pat. No. 3,110,483, issued to Baxa; U.S. Pat. No. 3,110,751, issued to Bade; U.S. Pat. No. 3,313,534, issued to Frans; and U.S. Pat. No. 3,653,645, issued to Heian et al. As disclosed in these patents, the problem in such systems is that of obtaining proper thermodynamic balance of heat inputs between the drying, preburning and final heating stages. This problem arises because for each material there are three requirements that establish desired temperatures within such systems. One requirement is that for each material there is an ascertainable BTU input and temperature level to which the material must be finally heated in the rotary kiln. A second requirement is that each material also has a known or ascertainable temperature level and total heat input that is necessary to achieve the desired preburn before the material is exposed to much higher temperatures in the kiln. A third requirement is that each material also has an ascertainable desirable maximum gas temperature for the drying stage so that water vapor is not produced so rapidly that the material being processed breaks into particle sizes so small that excessive dust is created.
The temperature of the gases that perform the final heating is a determining factor as to the temperature of the gases discharged from the kiln for preburning and drying material on the grate. Therefore, the degree to which the aforesaid first requirement is achieved affects the degree to which the second and third requirements can be achieved. Thus, the problem of proper thermodynamic balance between the drying, preburning and final heating stages is a continuous one because the gas flow begins with a specific volume of preheated gas from the cooler mixing with burning fuel in kiln to meet the first requirement.
Since thermodynamic balance, in systems of the type herein considered, is a continuing problem, any cement raw material which includes materials which introduce additional volatiles into the stages introduces factors which further affect the thermodynamic balance of heat inputs in the various stages of the system. Thus, kerogen containing cement raw materials has not heretofore been successfully processed in grate preheater kiln systems since the combustile volatiles from the kerogen materials has been too high for the systems to operate with. Another problem associated with cement raw material containing kerogen is the unusually high sulphur content. Thus, the gases from the processed material must also be treated to remove the sulphur from the waste gases.
The present invention is directed to the problem of processing kerogen containing cement raw material through a grate preheater kiln system, providing controlled thermodynamic balance in the system, and reducing the amount of external fuel requirements relative to the tonnage of material treated.
Still another object of the present invention is to provide a treating method to process sulphur-laden gas given off by kerogen materials contained in cement raw materials to prevent discharge of excessive sulphur into the atmosphere.